At a blast pressure of 100 psi, a grain of abrasive shooting from the mouth of a wide-throat long venturi blast nozzle hits its target at 420 miles per hour. The velocity increases to approximately 850 miles per hour --faster than the speed of sound --at 150 psi. The Occupational Health and Safety Organization (OSHA) requires a safety system on all abrasive blasting equipment as well as on other equipment. Such safety systems usually include what is referred to in the art as a "deadman's control." A deadman's control is a device that stops the machinery when the control is released. As is well known, these controls have been implemented as mechanical, pneumatic and electric deadman controls. To insure worker safety, these deadman's controls should be reliable, safe, and activate under of a variety of predictable and unpredictable conditions, such as accidental start-up, operator error, etc.
As is well known, deadman's controls may be implemented at different locations within a system, such as an abrasive blasting system. One such implementation is to put the device which the deadman's apparatus actuates at a point in the system near the so-called mixing pot. In such an arrangement, when an operator holding a hose releases a deadman's control at or near a nozzle end, the device under its control at the mixing pot shuts flow to the hose. With such arrangement, there is typically a delay of several seconds before spray in the hose is shut down, resulting in continued flow at the nozzle, which then may lead to significant operator injury.
What is needed is a simple and effective deadman system and method to insure that flow is redundantly terminated without undue delay under a variety of conditions so as to provide operator safety.